1. Technical Field
The present invention relates generally to photoresists used as a mask for etching purposes, and more particularly to a method of preparing reduced photoresist critical dimensions by forming a hardened top photoresist layer and performing an isotropic etch procedure.
2. Background Art
The process of accurately etching patterns has been the subject of significant development, particularly in the field of semiconductor electronics. The degree of circuit miniaturization, affecting a product's size and operational frequency limits for example, depend on the degree of accuracy of etching. The process of etching a pattern involves the use of a mask to selectively allow an etchant to remove the semiconductor or conductive material, as required to form the desired pattern. Typically, a mask is formed by spin coating a layer of liquid photoresist on the material to be etched. The desired pattern on the photoresist is then exposed to a form of radiation, for example, through use of an optical mask and ultraviolet light. The exposed areas of the photoresist are rendered either soluble or insoluble to a developer, depending on whether the resist is a positive or negative type. The soluble portions are then removed, and the remaining photoresist functions as a mask for selectively allowing an etchant to remove underlying material in areas void of photoresist protection.
A particular problem arises when the creation of resist line widths of small dimensions is required. The minimum resist line width that can be formed in a direct application of photoresist and subsequent etch is often not small enough. For example, the formation of transistor gates of length (actually a line width) less than 0.15 microns is very difficult, requiring expensive UV and/or phase shift methods. A simpler technique that is used is called "gate trimming", wherein after the lithography (photoresist pattern) is formed on the substrate, the resist is etched down so as to narrow the line widths. This technique has severe limitations, as will be explained in reference to FIGS. 1A and 1B.
FIG. 1A shows a cross section of a photoresist line 10 of width "w" and height "h". In order to further reduce the width "w" prior to performing an operation on a substrate 12, a selective isotropic etch procedure is performed on the resist 10. The result is shown in FIG. 1B. The width "w" has been reduced to "w.sup.1 " at the cost of losing much of the protective height "h" that is reduced to "h.sup.1 ". Since there are limits to the ratio of "h" to "w" in FIG. 1A that can be realized, there is a corresponding limit to the width "w.sup.1 " that can be achieved by etching, before the height "h.sup.1 " is too small to provide adequate protection to the underlying area 14 from subsequent processing steps.
From the above discussion, it is apparent that there is a need for an improved method of preparing narrow photoresist line widths.